Magnetic trapping of spin-polarized atomic hydrogen

We have confined over 5×{}10¹² atoms of hydrogen in a static magnetic trap. The atoms are loaded into the trap by precooling with a dilution refrigerator. At operating densities near 1×{}10¹³ cm^-3 the gas is observed to be electron and nuclear polarized in the uppermost hyperfine state. The long lifetime of the trapped gas (over 20 min) suggests that it is thermally decoupled from the wall and has evaporatively cooled to a temperature of about 40 mK. The residual decay of the gas density is consistent with spin relaxation induced by dipolar interactions between atoms.